Method of pretreating pulp in an acid tower prior to bleaching with peroxide

ABSTRACT

The present invention relates to a method of pretreating, by acidification and chelation, of pulp to be bleached with peroxide. Especially, the invention relates to intensifying, and at the same time simplifying the pretreatment required by peroxide bleaching. It is characteristic of the invention to introduce pulp into an acid tower ( 14, 112 ) where the pressure is 0 to 20 bar, preferably 1 to 10 bar, and the temperature 75 to 130° C. preferably 80 to 110° C., to treat the pulp in acid tower ( 14, 112 ) at a pH of 2 to 6, preferably 3 to 4, for 20 to 240 minutes, for decreasing the kappa number by 1-9, preferably 2-6 kappa units, to transfer the pulp from acid tower ( 14, 112 ) to a tower ( 24 ) of a second treatment stage, to treat the pulp with either a complexing agent at a pH of 4 to 9, preferably 5 to 6, or with an oxidizing chemical such as chlorine dioxide, Caro&#39;s acid, peracids or equivalent, and to wash and/or press the pulp.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of bleaching pulp withperoxide. The invention especially relates to intensifying and at thesame time simplifying the pretreatment required by peroxide bleaching.

Bleaching of pulp with peroxide is previously known in many connections.Especially in bleaching of chlorine-free pulp, peroxide has an importantrole. Prior to peroxide bleaching, it is necessary to remove heavymetals from the pulp by utilizing, for example, complexing agents suchas EDTA or DTPA. It has been established in tests that a suitable pHvalue is 4 to 7, preferably 5 to 6, in this so-called chelating stage.

On the other hand, it has been surprisingly established (FI 944808)recently that the kappa number of pulp may be decreased by mere acid ina pH range of 2 to 6, preferably 3 to 4. The temperature has to be 60 to130° C. and the duration 20 to 240 minutes in this, so-called acidifyingtreatment. A suitable acid is aminic acid, sulphuric acid, orhydrochloric acid, even though other corresponding acids may beconsidered as well. In other words, we have found that, besides peracidssuggested in Swedish patent 500605, which, as known, contain adelignifying perhydroxyl ion, a suitable acid may be some acid whichdoes not contain any known delignifying ion or equivalent. Aprerequisite for operation without peracids or equivalent is that thetemperature is high enough (cf. FI 944808). Peracids and equivalent donot call for high temperatures; usually a temperature below 75° C., mostusually that of 50 to 75° C., is sufficient. Treatment with acid may beintensified with additional chemicals, but it is once more to be notedthat it is not at all necessary for decreasing the kappa number. Suchadditional chemicals are those which make the treatment of metals moreeffective or more effectively decrease the kappa number.

It has been often thought that acidification (A) and chelation (Q)stages could be united, but practical experience has shown that it isimpossible. The pH ranges of acidification and chelation deviate fromeach other and, therefore, two separate treatment towers are necessary.At the acidification (A) stage, the pH has to be low enough, thetemperature high enough and the treatment time long enough. At thechelation stage (Q), however, the pH has to be high enough. It is anobject of this invention to provide a method of implementingacidification, decrease of the kappa number, and chelation, as simplyand efficiently as possible, excluding unnecessary pumping operations.

It is previously known that prior to peroxide bleaching, the pulp istreated at a ZQ stage where, at the Z stage it is delignified with ozoneat a pH of 2 to 4 and, thereafter, treated at the Q stage for removingmetals. However, there is a problem of the Z stage being fast, usuallytaking less than 1 minute, and often cold, below 70° C. The Z stagethereby provides poor conditions for dissolving metals from fibers. Thismay be partly remedied by adding an A stage prior to the ozone treatmentso that an AZQ stage, i.e., an arrangement with three towers, is broughtabout. This arrangement involves two problems in view of dissolvingmetals. One is the temperature and the other concerns removal of heavymetals; the best way of removing heavy metals is to provide extractingtime for pulp after delignification. In other words, with regard tometals removal, the ZQ stage should be an AZAQ stage, i.e., anarrangement with four towers, which should be run at a high temperatureof preferably over 70° C. But a further problem is involved in here,namely the ozone treatment produces radicals which are harmful to thepulp quality and which have enough time so as to react with pulp in thesecond A tower.

In Tappi Pulping Conference held in 1994, Nordgren and Elofson suggestedin their paper “New process for metal ion chelation at elevated pH inpulp production” including an AQ stage for removal of metals. Accordingto their teaching, the pH is 3 to 5 at the A stage and 6 to 9 at the Qstage. They suggest that the temperature of the process be 75° C., whichis too low in view of decreasing the kappa number. A weakness of themethod by Nordgren and Elofson is thereby that after their AQ stage, thebleachability of the pulp is still rather poor since the kappa numberhas not been lowered at the A stage, which impairs dissolving of metalsand weakens the bleachability.

In the 1994 Tappi Pulping Conference, the article “Metal management inECF bleaching and the effect of peroxide efficiency in the EPO stage”studied the effect of chelating agents when these were added to thechlorine dioxide stage. It is established in the study that, chelatingagents, when added to the chlorine dioxide stage, do not lessendelignification at the chlorine dioxide stage, but delay the increase ofbrightness instead. The EPO stage functions better if chelating agentsare added to the chlorine dioxide stage preceding the EPO stage. So, itis suggested in the article that the D_(Q)E_(PO) sequence may beimproved by converting it into a DQE_(PO) sequence, by adding onetreatment stage, i.e., a Q stage. The research had been made using thetemperature of 60° C. at the D stage, which is too low in practice. Thekappa number was also too high, i.e., nearly 30.

It is a characteristic feature of a preferred embodiment of the methodof the present invention that, prior to the peroxide stage which ispreferably pressurized, most preferably a pressurized two-tower peroxidestage, pulp is treated at a two-tower treatment stage where the kappanumber of pulp is lowered in acidic, hot conditions, the pH beingpreferably 2 to 6 and the temperature 75 to 130° C., and thereafter at achelation stage, the pH being 4 to 9 preferably 5 to 6. Hereby, thebleachability of pulp is made optimal with regard to both metals and thekappa number prior to the peroxide stage.

It is a characteristic feature of another, alternative embodiment of themethod of the invention that pulp, the kappa number of which has beenlowered with a hot acid treatment, is treated with chlorine dioxide orsome peracid at the same bleaching stage, but in a separate tower.

The characteristic features of the method of the invention appear fromthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an installation according to a preferred embodimentof the invention,

FIG. 2 illustrates an installation according to a second preferredembodiment of the invention, and

FIG. 3 illustrates an installation according to a third embodiment ofthe invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment of FIG. 1, pulp is transferred from a precedingtreatment stage 10 by an MC® pump 12 to an acid tower 14. The precedingtreatment stage may be oxygen delignification, which most usuallyfollows pulp cooking, washing subsequent to that, or some otherdelignification or bleaching stage, or washing subsequent to that.Before tower 14, necessary chemicals are added to the pulp and, ifnecessary, steam is added for raising the temperature. These chemicalsare the acids mentioned above (e.g. hydrochloric acid, sulphuric acid,or aminic acid, i.e., acid which does not contain oxidizing perhydroxylion) and, e.g., enzymes, magnesium and/or calcium, which are added asMgSO₄ and/or CaO. It is possible to add the chemicals either directly topump 12, to inject them into a tube 16 between pump 12 and tower 14, orinto a mixer 18 specifically arranged for this purpose. The conditionsin the acid tower 14 are as follows: pressure 0 to 20 bar, preferably 1to 10 bar; temperature 75 to 130° C., preferably 80 to 110° C.; and pH 2to 6, preferably 3 to 4. Treatment in the acid tower takes 20 to 240minutes, preferably 45 to 150 minutes. The kappa number usuallydecreases by 1-9 units, most usually by 2-6 units, in the acid tower.

After the acidification, chemicals needed in chelating are added to thepulp. Such chemicals are a complexing agent, e.g., EDTA and DTPA, andpotentially metals, such as magnesium and lime. Metals may advancechelation. Enzymes may be used as well. Chelation purposes to removeheavy metals which catalyze degradation of hydrogen peroxide, such asmanganese and copper. A suitable pH value for chelation is 4 to 9,preferably 5 to 6. If the pH after acidification is in a range which isunsuitable for chelation, the pH is adjusted to a suitable value byadding either acid or alkali (NaOH). Addition of NaOH is usuallynecessary in order to raise the pH from the level prevailing inacidification. A suitable place for adding these chemicals is adischarge means 20 of the acid tower, i.e., a so-called A tower, or asubsequent transfer line 22, between the acid and chelating towers 14,24. A portion of the chemicals may be added already in the A stage,e.g., MgSO₄, EDTA, DTPA. The main thing is that they are present whenthe Q stage begins and the pH is raised.

By the pressure developed by A tower 14, or actually by feed pump 12,pulp is transferred to Q tower 24. Thus, the A stage and the Q stage usetwo separate towers 14 and 24 without any pumping needed between thetowers. Pulp discharge means 20 may be so designed that it mixeschemicals and/or raises the pressure. In FIG. 1, A tower 14 is an upflowtower as well as Q tower 24. Towers 14 and 24 may be either of upflow ordownflow types, depending on the circumstances. In some cases, pulp mayalso be transferred by mere gravity from the A tower to the Q tower.

In Q tower 24, pulp is chelated. The conditions are as follows:retention time at least 10 to 60 minutes at the pH of 4 to 9, preferably5 to 6. The temperature and pressure have not been established to havemuch effect on the chelating process, even though the Q tower may bepressurized. The treatment time in Q tower 24 is not critical either,but it may be even several hours longer than that mentioned before, forexample, when Q tower 24 is used as a storage tower for pulp, i.e., aconventional high consistency pulp tower.

After Q tower 24, pulp is washed and/or pressed. Pressing refers to awashing method, in which liquid and various substances dissolved andextracted therein in acidification and chelation of pulp and produced bydifferent reactions are pressed from the pulp coming from, e.g., tower24 at a medium consistency of 10-14% so that the pulp consistency risesto >30%, whereafter the pulp is diluted back to the medium consistencyrange. FIG. 1 shows a washer 28, whereinto pulp is discharged from tower24 either by pressure of tower 24 or by a bottom discharge means 26which raises the pressure; in any event without a separate pump. Washer28 is preferably a so-called fractionating washer, which means thatseveral filtrates of different consistencies are obtained from the samewasher. A so-called DRUM DISPLACER® washer disclosed, e.g., in U.S. Pat.Nos. 4,919,158 and 5,116,423 is a fractionating washer. One of thefiltrates produced in washer 28, preferably filtrate F1, rich in heavymetals, is removed via a tube 30 and the other, F2, is returned via atube 31, for example, to a washer preceding the A stage. Thus, the AQstage is partly closed.

After washing with washer 28, the pulp is bleached with peroxide. It isadvantageous to use a pressurized reactor, especially a twin vesselreactor, as illustrated in FIG. 1. Irrespective of the reactor type, asuitable peroxide dosage is 5 to 20 kg of H₂O₂/adt and, depending on thekappa level, 0 to 15 kg/adt of oxygen, preferably about 5 kg/adt may beadded. Also alkali is added to the peroxide stage, for raising the pHand, if necessary, magnesium is added, for example, in form of magnesiumsulphate. The temperature is 90 to 130° C. The peroxide stage may bepreceded by some other bleaching stage, e.g., ZQ stage. On coming to theP stage, the pulp has a kappa number which is preferably below 10, oftenbelow 6.

In the method utilizing a twin vessel reactor, pulp is pumped at amedium consistency with MC® pump 32 via mixer 34, if desired, to apretreatment reactor 36, which is designed for a treatment time of 10 to60 minutes. The pressure in the reactor is 3 to 20 bar, preferably about10 bar. Bleaching chemicals H₂O₂ and oxygen are fed to pump 32 or mixer34. The peroxide dosage is 5 to 20 kg/adt, preferably about 10 kg/adt.The oxygen dosage is usually 0 to 15 kg/adt, preferably about 0 to 10kg/adt, most preferably about 3 to 5 kg/adt. The temperature is 80 to110° C., preferably 90 to 100° C.

In pretreatment reactor 36, peroxide reacts quickly and, after about 30minutes, 75% of the peroxide has been consumed. This means also that 75%of the reaction gases have been generated. Therefore, a gas separator 38is installed on top of pretreatment reactor 36, for separating gas fromthe pressure space of reactor 36. The separating capacity of thisseparator 38 is 40 to 90% of the gas volume contained in the pulp. Thepulp which is still under pressure after gas separation is taken via atube 40 to the bottom of the bleach tower 44 itself, where the pulpflows upwardly by its own pressure, with-out a separate pump. The bleachtower 44 need not necessarily be pressurized; any existing tank of asuitable size is applicable. However, it is advantageous to maintain aslight over-pressure, i.e., 1.1 to 5 bar, in the bleach tower. Asuitable retention time in tower 44 is 30 to 200 minutes. Additionalchemicals may be applied on pulp between towers 36 and 44 either viamixer (not disclosed) or, for example, by injecting them. After thebleaching reactions have taken place, the pulp has ended up in the topsection of tower 44, and flows as a result of height difference to thefollowing treatment stage, without a pump. As tower 44 is pressurized,its discharge opening may be provided with a gas separator 46, forremoving gases formed by the peroxide reaction, and if the gas separatorraises the pressure, the additional pressure generated by it may beutilized for further feeding of the pulp.

The most preferred sequences applying the method of the invention arethe following:

Cooking - O - AQ - P (brightness over 80), Cooking - AQ - ZP (brightnessover 83), Cooking - O - AQ - P - AQ - P (brightness over 83), Cooking -O - AQ - P - ZQ - P (brightness over 88), Cooking - O - AQ - ZQ - P - ZP(brightness over 88), Cooking - O - AQ - P - ZP (brightness over 88),and Cooking - O - AQ - ZQ - P (brightness over 85),

in which P may be an oxygen-reinforced peroxide stage P_(o) in which theperoxide dosage is over 5 kg H₂O₂/adt, preferably 5 to 20 kg H₂O₂/adt,and the oxygen dosage 0 to 10 kg O₂/adt, or a peroxide-reinforced oxygenstage O_(p), in which the peroxide dosage is below 10 kg H₂O₂/adt andthe oxygen dosage over 5 kg O₂/adt, preferably 5 to 15 kg/adt. In asequence comprising several peroxide stages P, the first P stage shouldpreferably be a peroxide-reinforced oxygen stage O_(p) and the second Pstage an oxygen-reinforced peroxide stage P_(o). The peroxide stage mayalso be an acidic P stage P_(a), whereby bleaching is effected, e.g., byCaro's acid or peracid. In other words, the sequences may also be thefollowing:

Cooking-O-AQ-P_(a)-AQ-P,

Cooking-O-AQ-P_(a)-ZQ-P,

Cooking-O-AQ-ZQ-P_(a)-ZP, or

Cooking-O-AQ-P_(a)-ZP.

The above-identified sequences may be simplified by leaving out washers.Washing before the A, Q, or AQ stages is not always necessary f or theprocess, even though the consumption of acid increases, but often thisis not too expensive in comparison with the washer price. Therefore,marking “-” which usually indicates washing and/or pressing, may be leftout before the A and/or Q stages. Thus, e.g., the partial sequence P-AQis replaced with a partial sequence PAQ, or possibly with PA or PQ.

Thereby, e.g., the following sequences may also be considered:

Cooking - O - AQ - PQ - P (brightness over 88), Cooking - O - AQ - PQ -ZP (brightness over 88), Cooking - O - AQ - ZPQ - ZP (brightness over88), and Cooking - O - PAQ - P (brightness over 85)

As in the above-mentioned sequences, PQ may also in these sequences beP_(a)Q, whereby P_(a) means acidic peroxide stage, i.e., treatment with,e.g., Caro's acid or peracid.

The following exemplary sequences are also possible:

Cooking-O-AQ-P_(a)Q-P,

Cooking-O-AQ-P_(a)Q-ZP,

Cooking-O-AQ-ZP_(a)Q-ZP, or

Cooking-O-AP_(a)Q-P.

Another simplification, which is worth while sometimes, is to replaceAQ, PQ, P_(a)Q, or ZQ with either A, P, P_(a), or Z. This can be donewhen metal removal is even otherwise sufficient. When there are two Qstages in the sequence, one of them, preferably the first one, maysometimes be left out. The kappa number after cooking is 35-15 or evenless. At the oxygen stage, pulp is delignified to a kappa number below20, preferably below 10. So, it is typical to a method according to apreferred embodiment of the invention that the method is applied to apulp, the kappa number of which has, by cooking and potentially also bydelignification, been brought to a value below 20, preferably below 10.

The A stage may be intensified by adding some chemical which advancesbleaching or some bleaching chemical to it. It may be, for example, someenzyme or chlorine dioxide. It has to be noted, however, that theabove-mentioned decrease of the kappa number by 1-9 units at the A stagemay be reached without the additional chemicals mentioned here, so, theadditional chemicals are only used for making the decrease of kappanumber more efficient. Then, the A stage may be A_(Enzyme) (A_(E)) orA_(Dioxide) (A_(D)). When enzyme is added to the A stage, a suitable pHis 4 to 5, and a suitable temperature 70 to 90° C. When chlorine dioxideis added to the A stage, a suitable end pH is 3 to 5 and the initial pHa little (2 to 4 units) higher. A suitable temperature is 80 to 100° C.when dioxide is used. If chlorine dioxide is added, it may be worthwhile destroying the chlorine dioxide residuals with SO₂ or NaOH beforeadding the chelating agent, to prevent the chelating agent from becomingdestroyed. In the above-identified sequences, A may thereby be A_(E) orA_(D) or some other intensified A stage.

EXAMPLE

In test runs, both hardwood and softwood pulps were cooked anddelignified to a kappa number of approx. 10. Thereafter, the pulps weretreated at an acid stage, where the temperature was 100° C., pH 3 to 4,and treatment time 3 hours. After acid treatment, the pulps were treatedwith EDTA at the pH of 5.5 to 6.5. After this, the kappa numbers weremeasured. The kappa number of hardwood pulp ranged from 7 to 5 and thatof softwood pulp from 8 to 6.

When chlorine dioxide was added to the acid stage, it was possible tofurther reduce the kappa number by 1-4 units. It was established in thetest that a suitable dosage was 5 to 30 kg, preferably 10 to 20 kg, ofchlorine dioxide per pulp ton calculated as active chlorine.

After the acid stage, the pulps were bleached at a pressurized peroxidestage, and this AQ-P treatment resulted in brightness values of over 85.In other words, it could be established that, when the treatment wasstarted with a pulp having a sufficiently low kappa number, thebrightness values obtained were clearly higher than those mentioned inthe above-identified sequences.

When bleaching was carried out with the A_(D)Q-P combination, thebrightness values of over 88 were obtained the dosage of chlorinedioxide being 10 to 20 kg/ADMT.

During continued test runs, it was surprisingly found out that whenchlorine dioxide was not added directly to the A stage but the kappanumber was first allowed to drop by mere acid treatment, and chlorinedioxide was added only after that, this gave a better final result. Inother words, AD is a more efficient treatment than A_(D) and gives akappa number which is 2 to 4 units lower. With a partial sequence ADQ-P,the chlorine dioxide dosage being 10 to 20 kg/adt calculated as activechlorine, a brightness of 89 to 90 ISO was achieved.

A corresponding phenomenon was discovered when peracids, such asperacetic acid and Caro's acid, were used. With an acidic P_(a) stage, agood result is obtained with the partial sequence AP_(a)-P or AP_(a)Q-P.The explanation to this is that, at the A stage are removed alsohexenuronic acids, which, unless they were removed, would consumechlorine dioxide, peracetic acid, Carols acid, ozone, and otherbleaching chemicals. In other words, in view of the overall economy, itis advantageous that also hexenuronic acids are removed at the A stageand that the oxidizing chemical is added thereafter, the additionthereof being a treatment stage of its own.

The bleaching stage illustrated in FIG. 2, being in accordance with analternative embodiment and being used in the above-mentionedcontinuation tests, includes a pump 110, preferably a so-called MC®pump, for pumping pulp, which is preferably in a medium consistency,from some preceding treatment stage, for example, a washer 108 or apress, to a first treatment tower 112. From tower 112, pulp isdischarged, preferably but not necessarily, via a top discharge means toa second treatment tower 122. If the top discharge means is used, it maypreferably be such that it raises pressure to some extent, 0.1 to 10,preferably 1 to 5 bar, so that the pressure generated by it may be usedfor transferring pulp from one tower to another. The discharge means 114may also be provided with gas separating devices in accordance with A.Ahlstrom Corporation's patent applications PCT/FI90/00085 orPCT/FI92/00216. It is a characteristic feature of the embodiment shownin FIG. 2 of the invention that the first treatment tower 112 isintended for acid treatment (A), whereby the acid (preferably sulphuricacid or some organic acid such as aminic acid) is fed and mixed with thepulp, preferably in pump 110. The need for acid may be considerablydecreased by bringing filtrate to the washer 108 preceding the describedAD stage from the washer 126 (illustrated by a dashed line in FIG. 2)subsequent to the AD (acidic) stage. So, acid is only needed foradjusting the pH to a value as exactly desired. When pulp is beingdischarged from tower 112, alkali is mixed with it, for adjusting the pHof the pulp, either in top discharge means 114 or thereafter or in aseparate mixer. Preferably, a pipeline 116 combining the towers 112 and122 is provided with a mixer 118, by which chlorine dioxide is mixedwith pulp. In other words, tower 122 is a chlorine dioxide tower. Thepurpose of the chlorine dioxide is to activate pulp for furtherbleaching treatments.

So, the above treatment stage is composed of two phases, A and D,carried out using two different chemicals. The first phase may becalled, for example, an acid phase. Its purpose is to improve thebleachability of pulp, and it is typically conducted in the followingprocess conditions:

consistency 8 to 20%

temperature 80 to 110° C.

pH 3 to 5, and

treatment time 30 to 120 min, whereby

the kappa number of pulp decreases by 1-6 units in said A phase.

It has been established on the basis of laboratory tests that withsoftwood pulp, the decrease of kappa number is about 1-3 units and withhardwood pulp about 2-6 units. Pulps of both type have been cooked andthereafter oxygen-delignified so that the kappa number is below 18,preferably below 12. On the basis of the tests, it has been establishedthat the A stage is most advantageous for a pulp pretreated in thismanner.

A second phase of the AD stage is D, and its process conditions aretypically as follows:

consistency 8 to 20%.

temperature 70 to 100° C. (may be higher)

initial pH 6 to 9

final pH 3 to 5

treatment time 10 to 180 min

chlorine dioxide dosage 5 to 30 kg ClO₂O/adt, and

chemicals adjusting the metal profile, such as Mg, Ca, EDTA, DTPA etc.,may be used in either the D phase or, e.g, thereafter.

It has to be noted that the A and D phases, both those described aboveand those to be mentioned later, may be carried out in reverse order,i.e., all AD stages or AD partial stages may be implemented in order DA,their effect being in that case, however, probably weaker.

Sequences applying the AD stage may be, e.g.:

O-AD-E-D, and O-AD-E-D_(E)-D, and O-AD-P_(o).

The oxygen delignification stage O may be left out if the kappa numberof the pulp coming from the cooking stage is sufficiently low.

According to a third preferred embodiment of the invention, use ofchlorine dioxide may also be included in a sequence which uses peroxideand chelation treatment prior to that. As known, chelation treatment (Q)means treating pulp with chelates (e.g., EDTA, DTPA or the like), whichtreatment is intended for removing heavy metals from pulp, such heavymetals being, e.g., iron, copper, and manganese so that they cannotdissolve peroxide. Suitable conditions for chelation treatmentincorporate pH of 4 to 6, treatment time of 10 to 60 minutes andtreatment temperature of 60 to 100° C.

When the peroxide stage P is used, pulp is preferably first treated intower combination DQ or possibly ADQ, for removing heavy metals, asillustrated in FIG. 3. In some cases, particularly when chelating agentsare not desired to be used, the tower combination AD as shown in FIG. 2is used also for removing heavy metals. FIG. 3 illustrates threesuccessive towers, 112, 122, and 132. The first of the towers is an acidtreatment tower 112 and, as mentioned earlier, it is only used accordingto need (for example, if the kappa number of pulp has to be lowered). Asfor the second tower 122, the embodiment of FIG. 3 corresponds to FIG.2. However, from the second tower 122, pulp is discharged preferably toan open chelation tower 132, and after having been treated therein, pulpis cleaned of heavy metals by washing them off of the pulp in washer126. Also towers 122 and 132 may be provided with heat transfer surfaces120, whereby the temperature in different towers may be selected withoutany direct use of steam.

Sequences O-DQ-P_(o) or O-ADQ-P_(o), implemented with the abovedescribed arrangement of towers, give quite high brightness values.According to laboratory tests, the brightness values obtained by usingsaid sequences are over 85 ISO. Brightness may be further increased byadding D, Z, or P_(o) stages. Sequences which are longer and produce ahigher brightness value are thereby, e.g., O-D-E-DQ-P_(o), orO-DQ-P_(o)-DQ-P_(o). Mg, Ca, and other chemicals for making the metalprofile more even, may be added to the chlorine dioxide stage. In thisway, DQ may possibly be replaced by mere D, to which one or morechemicals, such as Mg, Ca, EDTA, DTPA, have been added. Thus, DQ maymean an intensified D stage with regard to metals treatment. It is alsoadvantageous to combine the A treatment with D stages in these sequencesso that the D stage is replaced with an AD stage and the DQ stage withan ADQ stage.

As can be seen from the above description, the pretreatment stagespreceding the peroxide stages according to the invention are very simpleand, on the other hand, also effective. As, for example, each treatmentstage (A and Q) is effected at a pH which is exactly as required by it,the efficiency of the stages is brought to a maximum. However, thepressurizing of the pretreatment stage/pretreatment stages has resultedin that the investments in equipment remain relatively low, because thenumber of pumps has been minimized. In comparison with some earliersuggestions, the method of the invention also brings a saving of atleast one washer, because earlier it was suggested to have a washingstage also between the acidification and chelation. It has to be noted,however, that only some preferred, exemplary embodiments have beendescribed above, and that they are by no means intended to limit thescope of our invention, which is presented in the accompanying claims.

What is claimed is:
 1. A method of pre-treating, prior to bleaching withperoxide, cellulose pulp, to improve bleachability of the pulp, using anacid tower, and a tower in a second treatment stage, comprising thesteps of substantially sequentially: (a) if necessary, adjusting the pHof the pulp to between 2-6 by adding aminic acid, sulfuric acid,hydrochloric acid or another acid which does not contain oxidizingperhydroxyl ions; (b) feeding the pulp to the acid tower; (c) treatingthe pulp in the acid tower at substantially the pH, between 2-6, towhich it has been adjusted in step (a), at a pressure of 0-20 bar, at atemperature of 75-130° C., and for 20-240 minutes, so that acidtreatment decreases the kappa number by 1-9 units; (d) transferring thepulp from the acid tower to the tower of the second treatment stage, (e)in the second treatment stage tower treating the pulp with a complexingagent at a pH of between 4-9; (f) washing, pressing, or both washing andpressing the pulp; and (g) bleaching the pulp with hydrogen peroxideusing 5 to 20 kg peroxide/adt and 0-15 kg oxygen/adt, wherein step (f)is practiced by washing the pulp in a fractionating washer so that afirst filtrate containing heavy metals is removed from the process, anda second, cleaner, filtrate is recycled for use in another stage of themethod.
 2. A method as recited in claim 1 wherein step (g) is practicedusing two towers which are different in size and connected to eachother, the first tower acting as a pretreatment reactor and the secondtower as a bleach tower.
 3. A method as recited in claim 2 wherein step(g) is further practiced by: mixing peroxide with the pulp; feeding thepulp into the pretreatment reactor and treating the pulp in thepretreatment reactor at a pressure of 3-20 bar and for a reaction timeof 10-60 minutes, so that the peroxide reacts with the pulp; separatinggas from the pulp; using the pressure in the pretreatment reactor,blowing the pulp to a lower section of the bleach tower so that the pulpflows upwardly in the bleach tower; and removing the pulp from the topof the bleach tower after the pulp reacts with the peroxide in thebleach tower.
 4. A method as recited in claim 3 wherein the mixing stepis practiced by adding 5-20 kg/adt peroxide, and 0-10 kg/adt oxygen tothe pulp.
 5. A method as recited in claim 3 wherein step (g) is furtherpracticed by using a peroxide dosage that is from about 5 to just below10 kg/adt, and using an oxygen dosage of between 5-15 kg/adt.
 6. Amethod as recited in claim 3 wherein during treatment of the pulp in thebleach tower the pressure is between 1.1-5 bar, and the temperature80-130° C.
 7. A method of pre-treating, prior to bleaching withperoxide, cellulose pulp, to improve bleachability of the pulp, using anacid tower, and a tower in a second treatment stage, comprising thesteps of substantially sequentially: (a) if necessary, adjusting the pHof the pulp to between 2-6 by adding aminic acid, sulfuric acid,hydrochloric acid or another acid which does not contain oxidizingperhydroxyl ions; (b) feeding the pulp to the acid tower; (c) treatingthe pulp in the acid tower at substantially the pH, between 2-6, towhich it has been adjusted in step (a), at a pressure of 0-20 bar, at atemperature of 75-130° C., and for 20-240 minutes, so that acidtreatment decreases the kappa number by 1-9 units; (d) transferring thepulp from the acid tower to the tower of the second treatment stage, (e)in the second treatment stage tower treating the pulp with a complexingagent at a pH of between 4-9; (f) washing, pressing, or both washing andpressing the pulp; and (g) bleaching the pulp in two stages usingperoxide, the first stage in sequence using a peroxide dosage of between5 to just below 10 kg/adt and with about 5-15 kg/adt oxygen, and thesecond peroxide stage in sequence having a dosage of 10-20 kg/adtperoxide and an oxygen dosage of 0-10 kg/adt.
 8. A method ofpre-treating, prior to bleaching with peroxide, cellulose pulp, toimprove bleachability of the pulp, using an acid tower, and a tower in asecond treatment stage, comprising the steps of substantiallysequentially: (a) if necessary, adjusting the pH of the pulp to between2-6 by adding aminic acid, sulfuric acid, hydrochloric acid or anotheracid which does not contain oxidizing perhydroxyl ions; (b) feeding thepulp to the acid tower; (c) treating the pulp in the acid tower atsubstantially the pH, between 2-6, to which it has been adjusted in step(a), at a pressure of 0-20 bar, at a temperature of 75-130° C., and for20-240 minutes, so that acid treatment decreases the kappa number by 1-9units; (d) transferring the pulp from the acid tower to the tower of thesecond treatment stage, (e) in the second treatment stage tower treatingthe pulp with a complexing agent at a pH of between 4-9; (f) washing,pressing, or both washing and pressing the pulp; and (g) bleaching thepulp with hydrogen peroxide using 5 to 20 kg peroxide/adt and 0-15 kgoxygen/adt, wherein steps (a) through (g) are practiced to produce pulpat an ISO brightness of over 88, and is a part of the treatment sequenceof Cooking-O-AQ-P_(a)Q-P.
 9. A method of pre-treating, prior tobleaching with peroxide, cellulose pulp, to improve bleachability of thepulp, using an acid tower, and a tower in a second treatment stage,comprising the steps of substantially sequentially: (a) if necessary,adjusting the pH of the pulp to between 2-6 by adding aminic acid,sulfuric acid, hydrochloric acid or another acid which does not containoxidizing perhydroxyl ions; (b) feeding the pulp to the acid tower; (c)treating the pulp in the acid tower at substantially the pH, between2-6, to which it has been adjusted in step (a), at a pressure of 0-20bar, at a temperature of 75-130° C., and for 20-240 minutes, so thatacid treatment decreases the kappa number by 1-9 units; (d) transferringthe pulp from the acid tower to the tower of the second treatment stage,(e) in the second treatment stage tower treating the pulp with chlorinedioxide and adding chemicals to the pulp to adjust the metal profile ofthe pulp prior to, or in combination with, the chlorine dioxidetreatment; (f) washing, pressing, or both washing and pressing the pulp;and (g) bleaching the pulp using hydrogen peroxide.
 10. A method asrecited in claim 9 wherein step (c) is practiced at a pH between about3-4, at a temperature of 80-110° C., for a time of 30-180 minutes, andso that acid treatment decreases the kappa number of the pulp by atleast 2 units.
 11. A method as recited in claim 10 wherein step (e) ispracticed using a complexing agent at a pH of between about 4-9.
 12. Amethod as recited in claim 9 wherein step (g) is practiced by treatingwith hydrogen peroxide alone in a stage, or by adding hydrogen peroxideto an alkaline stage.
 13. A method as recited in claim 11 wherein priorto step (e), between steps (c) and (e), adding acid or alkali to thepulp to adjust the pH thereof.
 14. A method as recited in claim 9wherein step (f) is practiced by washing the pulp in a fractionatingwasher so that a first filtrate containing heavy metals is removed fromthe process, and a second, cleaner, filtrate is recycled for use inanother stage of the method.
 15. A method as recited in claim 9 whereinstep (g) is practiced using two towers which are different in size andconnected to each other, the first tower acting as a pretreatmentreactor and the second tower as a bleach tower.
 16. A method as recitedin claim 15 wherein step (g) is further practiced by: mixing peroxidewith the pulp; feeding the pulp into the pretreatment reactor andtreating the pulp in the pretreatment reactor at a pressure of 3-20 barand for a reaction time of 10-60 minutes, so that the peroxide reactswith the pulp; separating gas from the pulp; using the pressure in thepretreatment reactor, blowing the pulp to a lower section of the bleachtower so that the pulp flows upwardly in the bleach tower; and removingthe pulp from the top of the bleach tower after the pulp reacts with theperoxide in the bleach tower.
 17. A method as recited in claim 9 whereinstep (g) is practiced in two stages using peroxide, the first stage insequence using a peroxide dosage of between 5 to just below 10 kg/adtand with about 5-15 kg/adt oxygen, and the second peroxide stage insequence having a dosage of 10-20 kg/adt peroxide and an oxygen dosageof 0-10 kg/adt.
 18. A method as recited in claim 9 wherein step (e) ispracticed by using 5-30 kg/adt chlorine dioxide calculated as activechlorine.
 19. A method as recited in claim 9 wherein steps (a) through(g) are practiced as part of a treatment sequence of the pulp, in whichthe steps are practiced to bleach the pulp to an ISO brightness of over80, comprising Cooking-O-AD-P, or Cooking O-ADQ-P.